Monocrotophos is both a systemic and contact pesticide, useful for a wide range of pests (Monocrotophos, In: The Pesticide Manual, ED. Worthing C. R. and Walker S. B., British Crop Protection Council, 1983, p 384). Pure monocrotophos is a colorless, crystalline material melting at 55.degree. C. Technical monocrotophos having a purity of approximately 78% is semi solid to solid at room temperature. Monocrotophos is freely soluble in water and also in any organic solvents except aliphatic hydrocarbons. It is thermally unstable at temperature greater than 38.degree. C. It decomposes in moist conditions and in the presence of short chain alcohols. It has acute oral lethal dose (LD).sub.50 (rat) 14 mg/kg and acute dermal LD.sub.50 (rat) 336 mg/kg. Technical monocrotophos and its formulations like 40% and 25% solution come under WHO class 1B i.e. `highly hazardous` class. Owing to its highly hazardous nature, there is a great need to encapsulate monocrotophos using a suitable polymer to reduce its oral toxicity. The present invention is, therefore, specifically pertains to a process for the preparation of polyurethane microcapsules containing monocrotophos.
Conventional pesticide formulations are generally available in the form of emulsifiable concentrate for foliar spray, granules for soil broadcast or wettable powder for seed-coat application. These conventional pesticide formulations, have certain drawbacks such as high mammalian toxicity, phytotoxicity and poor persistence due to environmental degradation which leads to its excessive applications. Therefore, a need exists for improving the formulation and application technology for existing pesticides in order to fulfill the needs of both the user and the environment.
Polymers have, traditionally, played an important role in agriculture (McCormick C. L., In: Encyclopedia of Polymer Science and Engineering, Vol. 1, 2.sup.nd Ed., Wiley & Sons, New York, 1984). Various types of controlled release formulations of pesticides possessing many desirable properties have been reported (Controlled delivery of crop-protection agents, Taylor and Francis Ltd., London, Wilkins. R. M. Ed., 1990).
Microcapsule is one of the controlled release forms, wherein an active agent (core material) is surrounded by a polymer film. This is achieved by a process called microencapsulation. Different techniques of microencapsulation are known which include phase separation, interfacial polymerization and mechanical methods such as spray drying. Number of reviews on microencapsulation techniques have appeared in literature (i) Madan P. L., Asian J. Pharm. Sci., 9, 1979, p1; (ii) Thies C. In: Encyclopedia of Polymer Science and Engineering, Vol. 9, Wiley & Sons, New York, 1987, p 724 and (iii) Porte H. and Couarrze G., In: Hand book of Powder Technology, 9 (Powder Technology and Pharm. Processes) 1994, p 513). U.S. patents disclosing various microencapsulation methods have been consolidated (Gutcho M. `Microcapsules and Microencapsulation Techniques`, New York, Noyes Data, 1976).
Interfacial polymerization is an important method among the various microencapsulation techniques (Arshady R. J. Microencapsulation, 6(1), 1989, p1-12 and 13-28) especially for microencapsulation of pesticides since high active agent/polymer ratio (between 0.5 to 0.95) can be achieved. Microencapsulation by interfacial polymerization involves (i) preparation of an aqueous or organic solution of the active agent containing monomer A (ii) dispersion and emulsification of this solution in a continuous phase containing monomer B (iii) polymerization at the interface between A and B, resulting in the formation of membrane of polymer AB enclosing active agent and (iv) decantation, centrifugation or filtration of the microcapsules formed. Polymers employed in microencapsulation by interfacial polymerization include polyamide, polyester, polyurea and polyurethane.
Microencapsulation of water soluble active agents:
There are several methods described in literature for microencapsulation of water soluble active agents viz. solvent evaporation (Huang H. P. and Ghebre-Sellassie., J. Microencapsulation, 6, 1989, p 219; Tabato Y. and Langer R., Pharm Res., 10, 1993, p 391), phase separation (Max E. Sufdy, J. Appl. Poly Sci., 27, 1982, p 4753) and interfacial polymerization (kil-Yeong C., Kyoung S. M. and Taihyum C., Polymer (Korea), 15, 1991, p 548). The fine sprayable matrix particles of water soluble insecticidal carbamates produced by DuPont to give wettable powder formulations involve either co-precipitation or co-melting of pesticide and polymer (Tocker S., In: controlled Delivery of Crop Protection Agents, Ed. R. M. Wilkins, 1990, p 261).
As monocrotophos is unstable in aqueous medium, phase separation and interfacial polymerization (using water in oil system) method cannot be employed for preparation of microcapsules. Interfacial polymerization methods, reported so far, employ oil in water or water in oil systems. There does not exist any reported microencapsulation method by inter facial polymerization which involves oil in oil system. The phase separation technique can be employed without use of water. However, this technique needs solvents in large volumes. Furthermore, monocrotophos is unstable at temperature higher than 38.degree. C. and readily reacts with amine. This severely limits the choice of polymer and available methods.
Although, microencapsulation using polyurethane as wall material is known in literature, it involves dispersion of organic phase consisting of isocyanate, glycol and active agent in an aqueous phase (i) Fuyuma H., shinjo G. and Tsuji K., J. Pesticide Sci., 9, 1984, p 511 (ii) Choi K.y., Min K. S., Park I. H., Kim K. S. and Chang T., Polymer (Korea), 14, 1990, p 392 (iii) Ohtsuti T., Polymer, 32, 1991, p 2395 (iv) JP 04, 76016 (v) Reddy P.V.S, Mahesh G. N. Ramesh S., Sivakumar P. A. and Radhakrishnan G., Macromolecular Reports, A32 (suppls. 5 & 6), 1995, p789 (v) JP 05, 194, 128 (CA 119:154041) (vi) JP 05, 201,814 (CA 119: 197763)). However, in view of the instability of monocrotophos in aqueous medium, the reported oil in water system described in above mentioned references is not applicable.